a) Field of the Invention
The invention is directed to a video projection device for showing a video picture composed of picture points of defined magnitude on a screen with at least one light source for emitting a light bundle with a predetermined beam cross section or beam profile for illuminating every picture point on the screen during a predetermined time interval. The invention is further directed to a method in which a video picture on a screen is constructed of picture points in that a light bundle with a predetermined beam profile is directed onto a screen to illuminate every picture point of defined magnitude during a predetermined time interval.
b) Description of the Related Art
Since its discovery, video and television technology has captured a considerable market in an impressive manner. Within a period of one year, approximately 60 million television sets are produced worldwide.
More recent developments have substantially improved picture quality with respect to sharpness, contrast and brilliance of color.
Above all, the picture reproducing element is decisive for picture sharpness. In television picture tubes, aperture masks or shadow masks, as they are called, are used to sharply delimit the individual pixels. The first task performed by these masks for displaying color is to separate the regions accessible to the three utilized electron beams for the phosphors on the screen to produce red, green and blue light. Secondly, a sharp framing or delimiting of the picture points is also effected by the aperture masks in the formation of a video picture. When a television picture tube is observed from a very short distance, the human eye can detect a pixel structure that is given by the localization of the picture points (pixels).
There are other methods for showing a video picture besides television picture tubes. Some of these methods make use of the projection of a primary image internal to the device. In so doing, the primary image is magnified on a screen by means of a projection lens.
Without doubt, the most commonly used apparatus of this type is the LCD projector. The primary image to be projected is formed by means of a liquid crystal matrix (LCD matrix). This liquid crystal matrix presents a fixed quantity of picture points, one LCD element being provided for each picture point. The division of the LCD matrix into individual LCD elements is clearly discernible on the projection screen especially as a result of the outer contour of the individual LCD elements.
The pixel structure is clearly discernible in a video projection device, likewise in a device proposed by Texas Instruments with tiltable mirrors for the illumination of picture points by means of the edges of the individual mirrors.
In image projection with "beamers", as they are called, the front surface of a television picture tube is projected onto a projection screen. In this case, again, the individual picture points are framed by the formation of the mask structure of the individual picture tubes.
In all of the methods mentioned above, the number of individual picture elements available for displaying picture points for constructing a video picture is determined by the construction of the corresponding picture reproduction systems. For this reason, these systems are generally only suitable for one television standard only. For example, a PAL television picture tube has 625.times.830 picture elements that are used for showing the picture points of a video system.
In the PAL display with liquid crystal matrices, the same quantity of LCD elements is required, just as the aforementioned mirror system must have the same number of mirrors.
Therefore, there is one inherent disadvantage in all of these systems. If a single picture element fails, the picture quality declines appreciably. Owing to the aforementioned large number of picture elements for showing the picture points, it is estimated that the probability for failure of a single picture element is very high.
This problem becomes even more relevant with regard to improving picture quality by means of an increased quantity of picture points, as is the aim in the HDTV standard, for example, in which approximately 3 million picture elements are necessary in any case. The high probability of failure has disadvantageous consequences above all in the aforementioned matrix with tilting mirrors by Texas Instruments. The development of this equipment has not yet yielded a marketable video device because the rejection quota is much too high.
Further, the techniques mentioned above have the drawback that video pictures can be displayed in only one standard. It is not possible to modify the displayable picture format, since, for example, the aspect ratios are also determined by the fixed standard.
The dimensions of a picture element can be determined in a simple manner from the picture format and the corresponding number of picture elements. These magnitudes vary between different standards. Accordingly, there can be no multivalent use of different standards and systems.
These disadvantages are overcome in the prior art according to the generic type. Such systems are known, for example, from DE 43 06 797 C1. According to this reference, a light bundle is deflected in a raster pattern on a screen, wherein the light intensity or light color of the deflected light bundle is modulated in a manner similar to that of the deflected electron beam in a picture tube for controlling a picture point.
Due to the raster scanning, every picture point is illuminated by the light bundle for only a short time in fractions of microseconds. In principle, the picture is first formed in the eye of the observer which, because of its inertia, averages the received light information over a period of more than 1/100second.
In these systems, there exists no internal picture within a projector. Picture elements delimited by physical apertures are not required in principle.
The size of a picture point is substantially determined by the diameter of the light bundle for illuminating the individual picture points. Therefore, because of the high degree of parallelism of the light, lasers are generally used for video projection devices of this type. In principle, LEDs, which are considerably cheaper than lasers, could also be used. However, the cost of parallelizing optics and increased output to compensate for light losses due to diaphragms and collimators would have to be taken into account.
There are firmly defined values for determining a spot illuminated by a laser. Since the intensity profile of the laser beam can generally be approximated by a Gaussian function and thus has no sharp boundary, the diameter is generally defined by a drop in intensity to 1/e.sup.2 with reference to the intensity maximum.
High quality must be preserved in the propagation conditions of the laser beam in order to achieve a required resolution demanded on the basis of the picture point size of a standard. The quality can be improved through the use of optical systems for beam focussing. In so doing, the outlay for optical equipment for achieving the required laser focus increases disproportionately with the resolution.